Structure of Water

How much of the Earth's surface is covered with water?

In his well-known poem “The Rime of the Ancient Mariner,” Samuel Coleridge wrote “Water, water everywhere, Nor any drop to drink.”  The narrator was talking about being out on the ocean, but not having any water because he had killed an albatross (apparently bringing bad luck to everyone on the ship).  About 75% of the Earth’s surface is water.  The major constituent of the human body (over 60%) is water.  This simple molecule plays important roles in all kinds of processes.

Structure of Water

Water is a simple molecule consisting of one oxygen atom bonded to two different hydrogen atoms.  Because of the higher electronegativity of the oxygen atom, the bonds are polar covalent (polar bonds).  The oxygen atom attracts the shared electrons of the covalent bonds to a significantly greater extent than the hydrogen atoms.  As a result, the oxygen atom acquires a partial negative charge (δ−), while the hydrogen atoms each acquire a partial positive charge (δ+).  The molecule adopts a bent structure because of the two lone pairs of electrons on the oxygen atom.  The H-O-H bond angle is about 105°, slightly smaller than the ideal 109.5° of an sp3 hybridized atomic orbital.

Polar molecules attract one another by dipole-dipole forces as the positive end of one molecule is attracted to the negative end of the nearby molecule.  In the case of water, the highly polar O-H bonds results in very little electron density around the hydrogen atoms.  Each hydrogen atom is strongly attracted to the lone-pair electrons on an adjacent oxygen atom.  These are called hydrogen bonds and are stronger than conventional dipole-dipole forces.

Have you ever been ice skating?

Ice is an interesting and useful material.  It can be used to cool food and keep it fresh.  It can provide recreation, such as ice-skating.  Ice can do great damage when it freezes – roads can buckle, houses can be damaged, water pipes can burst.   All this happens because of a unique property of water and ice.  When water freezes, it expands in volume as ice is formed.

Structure of Ice

Liquid water is a fluid.  The hydrogen bonds in liquid water constantly break and reform as the water molecules tumble past one another.  As water cools, its molecular motion slows and the molecules move gradually closer to one another.  The density of any liquid increases as its temperature decreases.  For most liquids, this continues as the liquid freezes and the solid state is denser than the liquid state.  However, water behaves differently.  It actually reaches its highest density at about 4°C.

Ice is less dense than liquid water and so it floats.  Ponds or lakes begin to freeze at the surface, closer to the cold air.  A layer of ice forms, but does not sink as it would if water did not have this unique structure dictated by its shape, polarity, and hydrogen bonding.  If the ice were to sink as it froze, entire lakes would freeze solid.  Since the ice does not sink, liquid water remains under the ice all winter long.  This is important, as fish and other organisms are capable of surviving through winter.  Ice is one of only a very few solids that is less dense than its liquid form.